In recent years, solid polymer-type fuel cell membranes are attracting attention for new energy sources, and polymer membranes for use as cation exchange membranes in fuel cells are being developed. Such polymer membranes have to show good proton conductivity and to be sufficiently stable chemically, thermally, electrochemically, and mechanically. Therefore, perfluorocarbonsulfonic acid membranes, typified by Nafion® manufactured by DuPont (U.S.A.), have been generally used as long-life practical polymer membranes. In the case that perfluorocarbonsulfonic acid membranes are used, however, there is a problem in which the water content of the polymer membranes can rapidly decrease under fuel cell operation conditions at a temperature higher than 100° C., and the softening of the membranes can be significant, so that fuel cells cannot deliver sufficient performance.
In order to operate fuel cells in a high temperature range of at least 100° C., fuel cell membranes of highly heat-resistant polymers are basically required. Thus, a variety of polymer electrolyte membranes in which a sulfonic acid group is introduced in aromatic ring-containing polymers have been investigated. On the other hand, it is conceivable that an acidic group may be introduced into aromatic polyazole polymers, such as polybenzimidazole, known as highly heat-resistant, highly durable polymers, for use in the above-mentioned applications. There is a report on polymer electrolyte membranes of sulfonic or phosphonic acid group-containing polybenzimidazole polymers (for example, see the brochure of International Patent Publication WO02/38650 (Patent Document 1)). These polymers do not have very high proton conductivity at or near 80° C. but are expected to have conductivity at high temperature. However, the increase in proton conductivity tends to be not so significant, though the polymers having a structure in which a sulfonic acid group is introduced as an acidic group have good solubility in organic solvents and thus have good workability. On the other hand, the polymers having a structure in which a phosphonic acid group is introduced as an acidic group tend to have higher proton conductivity when the acidic group content is increased, but such proton conductivity cannot be practically sufficient. These polymers also require humidified conditions to exhibit proton conductivity, and, therefore, it should be impossible to use them at a temperature of at least 100° C. without humidification.
Only the introduction of an acidic group such as a sulfonic or phosphonic acid group into polymers cannot produce practical proton-conductivity in a high temperature range of at least 100° C. under non-humidified conditions. Thus, there is reported a high-temperature fuel cell electrolyte membrane that is produced by impregnating polybenzimidazole with phosphoric acid so that an ion-conducting function is produced with the phosphoric acid (for example, see Japanese Patent Application National Publication (Laying-Open) No. 11-503262 (Patent Document 2)). Polybenzimidazole should be a polymer with high thermal stability, and there is a report that impregnation with phosphoric acid can further increase the thermal stability (for example, see E. J. Powers et al., High Performance Polymers: Their Origin and Development, Elsevier, N.Y. (1986), p. 355 (Non-Patent Document 1)).
However, polybenzimidazole itself has no ion conductivity, and, therefore, polybenzimidazole has to be impregnated with a large amount of phosphoric acid in order to produce sufficient proton conductivity. There is also a problem in which phosphoric acid is a low-molecular weight compound and thus can gradually leak from polybenzimidazole so that the ion conductivity can decrease over time. There is also a problem in which high phosphoric acid content can increase membrane swelling to interfere with a fuel cell assembling process. The literature also discloses the use of a polybenzimidazole structure produced with a dicarboxylic acid monomer having a pyridine skeleton, in which, however, the tetramine monomer used at the same time is only 3,3′-diaminobenzidine so that the polymer produced with such a combination tends to have the same defect as described above.
In contrast, the polymer in which a sulfonic acid group is introduced in polybenzimidazole as described above has an acidic group in its molecule and thus is expected to produce proton conductivity even with low phosphoric acid impregnation amount. Therefore, sulfonic acid group-containing polybenzimidazole polymer electrolyte membranes impregnated with inorganic or organic acids are reported (for example, see Japanese Patent Laying-Open No. 2003-327826 (Patent Document 3)). At present, however, even these membranes cannot deliver satisfactory performance, such as workability and fuel cell membrane properties, practically necessary for fuel cells. In these investigations, pyridine skeleton-containing polymer structures have not been investigated yet.    Patent Document 1: International Patent Publication WO02/38650    Patent Document 2: Japanese Patent Application National Publication (Laying-Open) No. 11-503262    Patent Document 3: Japanese Patent Laying-Open No. 2003-327826    Non-Patent Document 1: E. J. Powers et al., High Performance Polymers: Their Origin and Development, Elsevier, N.Y. (1986), p. 355